Electrolyte effects on surface chemistry of basaltic glass in the initial stages of dissolution

• Surface reactions of powdered basaltic glass were determined in a time period of 500 days.
• Response of basaltic glass surfaces on solution composition was studied by zeta potential (ζ).
• Over time a shift from negative to positive ζ was observed, most markedly in the presence of Ca2 + and Zn2 +.
• Positive ζ values on basaltic glass surfaces foster adsorption of anionic compounds and can have biological impact.
• Neutralization of deprotonated SiO− sites by monovalent cations can favor polymerization resulting in smaller Si release.

For an understanding of the effect of solution composition on the dissolution rate of basaltic glass detailed knowledge of surface chemistry is important. Here the zeta potential (ζ) as a characteristic parameter of the magnitude of surface charge at the solid–liquid interface was used to determine ionic effects on surface chemistry in initial stages of basaltic glass dissolution. In a systematic approach powdered synthetic basaltic glass was dispersed in solutions of different cations (NO3− salts of Na+, K+, Mg2 +, Ca2 +, Ba2 +, Zn2 +, and Al3 +) and anions (Na+ salts of F−, Cl−, I−, NO3−, SO42 −, C2O42 −, HPO42 −), each in concentrations of 0.1, 0.5, 1.0, 2.5, and 5.0 mmol/L. ζ was traced in time sequences up to 12,000 h at ideally circumneutral pH. Ion affinities to glass surfaces were characterized by sorption isotherms. A change of glass chemical composition by the formation of altered layers was determined by depth profiling using secondary neutral mass spectrometry (SNMS). The dissolution of the glass was quantified by the amount of Si released after 4000 h.A marked decrease of ζ in deionized water within the first 3 h reaction time is assigned to the desorption of alkali and alkaline earth metal cations from the glass surface and formation of negatively charged SiO− sites. The addition of anions resulted in stronger negative initial ζ values in comparison with the experiment in deionized H2O indicating marked anion adsorption on surface sites, most obvious for F−, C2O42 − and HPO42 −. The initial ζ was increased upon the addition of divalent cations indicating neutralization of negatively charged surface sites. Over time a striking shift from negative to positive ζ was obtained, most markedly for Ca2 + and Zn2 +. The addition of trivalent Al3 + resulted directly in positive ζ indicating a strong adsorption on glass surfaces. With the progress of the experiment the sign of ζ reversed to negative values again. The reason for charge reversal is not fully understood and might be related with cation adsorption exceeding the negative surface charge and a concentration of Fe oxides at the glass surface. After an ~ 2000 h reaction time ζ adjusted for most electrolyte additions to slightly negative ζ until the end of the experiment, indicating that a final state in the composition of surface sites was reached. The presence of monovalent Na+ and K+ in solution suppressed Si release from the glass, whereas it is accelerated by bivalent cations. It appears that the neutralization of deprotonated SiO− sites by monovalent cations – their preferential binding is also indicated by chemical analysis – favors polymerization resulting in slower Si release. Upon the addition of Al3 + it is likely that SiOAlOSi bonds are formed, which can suppress Si release. The presence of F−, C2O42 −, and HPO42 − clearly enhances glass dissolution, most probably by increasing the coordination of network forming cations, hereby weakening bonds. The observed generation of positive ζ on basaltic glass surfaces is remarkable, and can improve in natural systems the adsorption capability of the basaltic glass surface for negatively charged compounds from pore solution, anions, dissolved organic matter and also bacterial cell walls.